1. Field of the Invention
This invention pertains generally to high voltage vacuum insulators for particle accelerators and pulsed power systems, and more particularly to high gradient insulators (HGI's) formed from alternating layers of metal and dielectric, and most particularly to methods for forming same.
2. Description of Related Art
Various structures or devices for storing or transmitting electrical energy, e.g. capacitors, transmission lines, and accelerator components (e.g. Blumlein pulse generators), are constructed with pairs of conductors separated by insulators. These conductors generally form electrodes or transmission lines. For high voltages to be placed on these electrodes or transmission lines, the underlying insulator must not break down. To make the structures or devices compact, the components, both conductors and insulators, must generally be made as thin as possible, requiring high gradients across the insulators. This magnifies the problem of breakdown.
The electrical strength of vacuum insulators is thus a key constraint in the design of particle accelerators and pulsed voltage systems. Many potential applications of these systems require minimizing the volume occupied by the system, so there is always a drive to reduce the size of the insulator interfaces. Vacuum insulating structures assembled from alternating layers of metal and dielectric can result in improved performance compared to conventional insulators.
Therefore particle accelerator and pulsed power system design depends on the voltage-holding ability of their vacuum insulators. When subjected to strong electric fields, the vacuum insulators generally fail by surface flashover rather than through the bulk material. It has long been known that the electric field that can be sustained by an insulator scales as (length)−1/2. This suggests that a structure composed of thin dielectric layers would be able to withstand a higher field than a monolithic insulator of the same length and dielectric material, which is the basis of the “high gradient insulator” (HGI) concept. HGI's consist of alternating layers of dielectric and metal and have been shown to withstand gradients up to four times higher than conventional insulators.
Currently HGI's are prepared by machining or water-jet cutting laminated sheets of material to the desired shape. In this method, a laminated structure of sheets of material is first formed. The HGI is then cut out of the laminated structure. This is relatively expensive and results in the waste of material that is left in the lamination after the HGI's have been cut. It also limits the ability to tailor the geometry of the insulator surface in ways that could further increase the voltage-holding ability of these structures.
The prior art method uses alternating layers of dielectric, adhesive, and metal to form a stack, much like the layers of paper in a closed book. The stack is assembled, and then heated while pressure is applied to it. The final HGI's (usually right cylinders) are machined out of the stack by machine tools or water jets. This may work fine for very small stacks, an inch or less in thickness. However, machine tools or water jets have difficulty cutting through very thick stacks, so as the stack becomes thicker, more specialized . and expensive machines are needed to accomplish this.
The prior art process may also negatively affect the voltage-holding capability of the HGI's. When an insulator fails in vacuum, it usually fails by an electrical discharge along the insulator/vacuum interface (the insulator surface). The geometry of that surface is critical to the performance of the insulator. In the case of HGI's, the geometry of both the metal and dielectric layers is important. The surface structure of HGI's formed by machining or water jet cutting is known to differ from the ideal. It is also known that machining can cause deformation of the metal layers, which can lead to vacuum arcing. In addition, the machining process heats the material, which can lead to internal stresses in the insulator stack. These stresses could cause delamination of the HGI's, and probably also cause the dielectric layers to be recessed below the metal layers when the HGI's cool. The bottom line is that the machining and water jet cutting processes strongly affect the surface structure of the insulators in ways which are generally bad, but poorly-controlled.
Accordingly, it is desired to provide an improved method of forming HGI's.